Thermoplastic packaging films can be broadly grouped into two categories, one being rigid or semi-rigid films and the other being flexible films. The semi-rigid and rigid films are quite suitable for applications which require a thermoforming operation to form the film into a more or less fixed, self-supporting shape. Typically such films are used in blister packaging, skin packaging, and shallow tray forming for packaging luncheon meat and the like. In addition to being thermoformable, it is often desirable that a semi-rigid film be heat sealable and have a low oxygen permeability in order to oxygen from reaching oxygen sensitive food products. However, it is indeed rare that all of these desirable attributes will be found in a single layer of polymeric film so that in practice the desirable characteristics are built into a film by laminating together different polymeric materials each of which possesses one or more beneficial properties. One of the early film structures developed for high impermeability to oxygen was composed of a regenerated cellulose film coated with a vinylidene chloride copolymer which was subsequently laminated with or coated with a layer of an ethylene polymer to permit the making of hermetically sealed packages. In some later applications in which a cavity is formed in the film by a vacuum forming or a thermoforming process, film structures have been developed using biaxially oriented polyethylene terephthalate base film coated with a vinylidene chloride copolymer that is subsequently laminated or coated with a polymer of ethylene as a sealing layer. In other instances, where deeper drawn cavities are required a polyamide base layer has been substituted for the polyethylene terephthalate layer.
Wile polyvinylidene chloride and the copolymer of vinyl chloride and vinylidene chloride have been widely used because of their low oxygen permeability, another polymeric material has begun to attract attention because of its relatively low oxygen permeability. This material is the hydrolyzed ethylene-vinyl acetate copolymer. HEVA, an early description of which is set forth in U.S. Pat. No. 2,386,347 which was issued on Oct. 9, 1945 to John R. Roland, Jr. Films and shaped articles of HEVA were later described in U.S. Pat. No. 3,183,203 which was issued on May 11, 1965 to Yasuo Yoshimura et al, and a description of a laminate of HEVA and polyethylene is found in U.S. Pat. No. 3,540,962 which issued on Nov. 17, 1970 to Haruyoshi Anzawa et al. Still another example of a prior art laminate having a HEVA layer is found in U.S. Pat. No. 3,595,740 which was issued to Clare W. Gerow on July 27, 1971 where a laminate having a barrier layer of HEVA, an outer layer of a base thermoplastic polymer, and a heat sealing layer of an ethylene polymer is described as being suitable for deeply drawn thermoformed packages. However, none of the prior art discloses a multi-layer structure with a HEVA layer that has been stretched and oriented to become a relatively thin, flexible oriented packaging material. Accordingly, it is one object of the present invention to provide an oriented, flexible, multi-layer film having a layer of HEVA.
In the flexible film category, vinylidene chloride copolymers have also been widely used and have met with commercial success in making high abuse resistant multi-layer films with low oxygen pemeability. One example of a film of this type is disclosed in U.S. Pat. No. 3,741,253 which issued on June 26, 1975 to Harri J. Brax et al. While film structures having a vinylidene chloride copolymer layer do provide generally satisfactory oxygen pemeability and shrinkage, it is an object of the present invention to provide an alternate shrinkable material with oxygen barrier characteristics equal to and superior to the presently used vinylidene chloride copolymers.
One problem often encountered in prior art manufacturing processes for heat shrinkable, multi-layer film is that even with great care very small quantities of air can be entrapped between layers; and, when the film is stretched for orientation these small air bubbles are also stretched so that the area they cover is enlarged thereby not only detracting from the appearance of the film but providing focal points from which layer separation can occur. Accordingly, it is another object of the present invention to provide a process in which the problem of air entrapment between the layers of a multi-ply film structure is either eliminated or minimized.
In the prior art it is generally known that olefin polymers which have been cross-linked exhibit improved abuse resistance. Also, one of the successful commercial processes for cross-linking olefin polymers is by irradiation of the polymer with ionizing radiation. However, even though film laminates having a vinylidene chloride copolymer layer have been irradiated, such irradiation is not generally practiced as the vinylidene chloride copolymer may, in some instances, degrade and discolor when subjected to high dosages of radiation. Thus, in the prior art it is the usual practice to prepare and irradiate the substrate material before applying the vinylidene chloride coating with the result that only one layer of the film structure would be crosslinked. Therefore, another object of the present invention is to provide a process for making a heat shrinkable multi-layer film with an oxygen impermeable layer in which more than one layer of the film may be cross-linked.
In order for vinylidene chloride polymers and copolymers to be successfully extruded and worked as films and constituents of film structures, it has been found necessary in the prior art to include plasticizers and stabilizers in the vinylidene chloride extrudate. While the plasticizers achieve their purpose of making the film more workable they have the undesirable quality of increasing the oxygen permeability of the film. Accordingly, it is another object of the present invention to provide a multi-layer film with low oxygen permeability which does not require the use of plasticizers.
Still another object of the present invention is to provide a heat shrinkable film with a HEVA layer, a multi-layer film not previously available.
The foregoing and other objects are achieved by applicants' surprising discovery which is set forth in the Summary of Invention below.